This behavior is almost certainly not caused by Smart Charging.
If a Surface laptop only charges to 6% and then stops, that's abnormal. Smart Charging does not limit the battery to 6%.
Smart Charging typically limits charging to around 80%, not 6%.
It is designed to reduce battery wear when the laptop is plugged in for long periods.
When active, Windows/Surface usually shows a heart icon on the battery.
Restart the Device and Run the Surface Diagnostic Toolkit.
Run powercfg /batteryreport in an elevated Command Prompt — it'll show design capacity vs. full charge capacity, which will tell you fast if the battery itself is degraded.
Given it's 15 days old, I'd push for an exchange rather than troubleshooting it into acceptability.
I would recommend contacting Microsoft Support and requesting a replacement (DOA/early warranty exchange) rather than a repair. A new laptop should never behave this way.